The preparation of block copolymers is well known. In a representative synthetic method, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed, resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block copolymer so grown is living until terminated.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer non-reactive toward monomer or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. If the polymer is not terminated the living block copolymers can be reacted with additional monomer to form a sequential linear tri-block copolymer. Alternatively the living block copolymer can be contacted with multifunctional agents commonly referred to as coupling agents. Coupling two of the living ends together results in a linear triblock copolymer having twice the molecular weight of the starting, living, diblock copolymer. Coupling more than two of the living diblock copolymer regions results in a radial block copolymer architecture having at least three arms.
One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. Various block copolymers and processes for making them have been proposed over the years. Of particular interest are polymers of relatively high styrene content, which are useful for packaging and containers. Examples of such block copolymers, as well as the methods of preparing such block copolymers, include but are not limited to, polymers and methods disclosed in U.S. Pat. No. 4,925,899, U.S. Pat. No. 6,521,712, U.S. Pat. No. 6,420,486, U.S. Pat. No. 3,369,160, U.S. Pat. No. 6,265,485, U.S. Pat. No. 6,197,889, U.S. Pat. No. 6,096,828, U.S. Pat. No. 5,705,569, U.S. Pat. No. 6,031,053, U.S. Pat. No. 5,910,546, U.S. Pat. No. 5,545,690, U.S. Pat. No. 5,436,298, U.S. Pat. No. 4,248,981, U.S. Pat. No. 4,167,545, U.S. Pat. No. 4,122,134, U.S. Pat. No. 6,593,430, and U.S. Pat. No. 7,169,848.
While block copolymers are often used in compounded form, the presence of certain of the typical blending components can also have a detrimental impact on properties. Common blending components include plasticizing oils, tackifying resins, polymers, oligomers, fillers, reinforcements and additives of all varieties. Oils are often added to such block copolymers to increase softness and improve processability to the compound. However, such oils also typically reduce the strength and tear resistance of the compounds. What is needed now are new compounding materials that do not have such a dramatic negative effect on properties, while still imparting increased softness with improved processability.
Applicants have now discovered that, when certain low molecular weight anionic diene/vinyl aromatic oligomers or polymers are combined in a particular way with the styrenic block copolymers noted above, it is possible to obtain compounds having better strength and tear resistance than the analogous oiled compounds, and also experience significant improvements in manufacturing steps and economies as well as improved properties such as increased softness without a significant reduction in processability. In addition, such compositions have lower volatility at equivalent hardness, resulting in improved organoleptics, reduced fogging and reduced extractables.